A. Field of the Invention
The invention generally concerns the use of small pore size silicoaluminophosphate (SAPO) catalysts to produce light olefins from alkyl halides. In particular, the catalysts have a weak acid concentration of less than 0.55 mmol/g-cat, a total acid concentration of 1.5 mmol/g-cat or less, and exhibit improved stability and catalytic performance over prolong periods of use.
B. Description of Related Art
Descriptions of units, abbreviation, terminology, etc. used throughout the present invention are summarized in Table 1.
Light olefins such as ethylene and propylene are used by the petrochemical industry to produce a variety of key chemicals that are then used to make numerous downstream products. By way of example, both of these olefins are used to make a multitude of plastic products that are incorporated into many articles and goods of manufacture. FIGS. 1A and 1B provide examples of products generated from ethylene (FIG. 1A) and propylene (FIG. 1B).
Methane activation to higher hydrocarbons, especially to light olefins, has been the subject of great interest over many decades. Recently, the conversion of methane to light olefins via a two-step process that includes conversion of methane to methyl halide, particularly to methyl mono-halide, for example, to methyl chloride followed by conversion of the halide to light olefins has attracted great attention. Zeolite (e.g., ZSM-5) or zeolite type catalysts (e.g., SAPO-34) have been tried for methyl chloride (or other methyl halide) conversion. However, the selectivity to a desired olefin (e.g., propylene) and the rapid catalyst deactivation for the halide reaction remain the major challenges for commercial success.
One of the most commonly used catalysts in the petrochemical industry is ZSM-5 zeolite. It is a medium pore zeolite with pore size about 5.5 Å and is shown to convert methyl halide, particularly methyl chloride or methyl bromide, to C2-C4 olefins and aromatics under methyl halide reaction conditions. Whereas, molecular sieve SAPO-34, an isostructure of chabazite zeolite, having small pore opening (3.8 Å) is shown to convert methyl halide to ethylene and propylene and small amounts of C4 olefins. However, both catalysts are shown to deactivate rapidly during methyl halide conversion due to carbon deposition on the catalysts.
While the SAPO-34 catalyst has good selectivity for both ethylene and propylene, a major problem with the SAPO-34 catalyst is its lack of stable catalytic performance over prolonged periods of use for the alkyl halide conversion. Notably, the currently available SAPO-34 catalysts show methyl chloride conversion rates of less than 20% after being used for 20 h. Such deactivation of these catalysts require frequent or continuous catalyst regeneration, or frequent catalyst change-out resulting in inefficient plant operation, or use of more catalysts to produce the desired amounts of ethylene and propylene, which in turn increases the manufacturing costs. Still further, the catalytic material has to be re-supplied in shorter time intervals, which oftentimes requires the reaction process to be shut down. This also adds to the inefficiencies of the currently available SAPO-34 catalysts.
TABLE 1AbbreviationDescriptionÅAngstromAvg.AverageBET SABET (Brunauer-Emmett-Teller) surface area° C.degree Celsius° C./mindegree Celsius per minutecm3/mincubic centimeter per minggramg/cm3gram per cubic centimeterhhourm2/gmeter square per grammol %mole percentmmol/g-catmillimole per gram of catalystNH3-TPDammonia temperature programmed desorption%percentpsigpound per square inch gaugeSARsilica to alumina ratioXRFX-ray fluorescenceWHSVweight hourly space velocitywt. %weight percent